If a website wants me to watch some audio or video, it can serve up a file that VLC or some other external player can play, after I've been promoted to allow this to happen.
If you happen not to have a compatible video player installed on the machine that you are presently using, what message or prompt should the system display? Offer a chance to install a "codec pack"? That's what we had before Flash, and malware developers learned that it was effective to to disguise an installer as a codec pack or Flash Player update. And if a web application wants to play several audio streams at a time, such as a game that wants to play both music and sound effects, how would it "serve up a file" for each in an efficient manner?